An optical waveguide modulator, in which an optical waveguide is formed on a substrate plate comprising LiNbO3 (which may be referred to as LN hereinafter) or GaAs, is advantageous in that the working velocity is high, the dependency on wavelength is low and the driving voltage is low, and thus it is widely used for practical external modulators in the optical communication field. Particularly, an optical waveguide modulator is widely used in DWDM system in which the low wavelength-dependency thereof is advantageously utilized.
However, in the optical waveguide modulator, a drift in the working point voltage, which is referred to as a temperature drift or a DC drift, occurs. Due to this phenomenon, it becomes necessary that output light from the modulator is monitored and the monitored output is fed back to the working point voltage to maintain the working point in a particular one point on a particular characteristic curve even when the drift phenomenon occurs.
As a means for monitoring the output light of the modulator, a monitoring system in which an optical fiber for output light is connected to an optical coupler located outside of the modulator module, the outputted light is divided into a main signal light and a branch light for monitoring at the optical coupler, the monitoring branch light is converted to electric signal by a photoelectric conversion element, and DC voltage of the modulator is controlled in accordance with the electric signal, is known. In this system, however, there is the problem that since the optical coupler for providing the monitoring branched light and the photoelectric conversion elements must be arrange outside of the modulator module, the cost of the modulator system increases, limitations on the dimension and form of the system increase and the reliability of the system decreases.
As another means for monitoring the output light from the optical waveguide element, a system in which a coupler, particularly a directional coupler is arranged in the optical waveguide element and an output light waveguide portion for the monitoring light is provided in addition to a optical signal-outputting waveguide portion, is usually, utilized. In this system, an optical circuit for the branched monitoring light must be arranged in the optical waveguide element and an optical fiber for the monitoring output light, in addition to the optical fiber for the output light for signal, must be connected to the optical waveguide element.
As still another monitoring system, a system in which, as disclosed in Japanese Unexamined Patent Publication No. 11-194,237, an inclined hole is formed in a cladding portion on an optical waveguide, or a diffraction lens is arranged above an optical waveguide element, and a portion of output signal light in the optical waveguide is taken to the outside of the substrate plate of the element by the above-mentioned hole or lense, is known. In this system, installation of a lens etc. for taking the monitoring light out of the optical waveguide element is necessary and, as the monitoring light is taken out of the optical waveguide element, a member for receiving the monitoring light must be installed on the optical waveguide element after the element is fixed in a container case, and this installation is difficult and complicated.
Further, Japanese Unexamined Patent Publication No. 5-34,650 discloses a system in which an end of an optical waveguide element is formed into an inclined form, a portion of output light from the waveguide is reflected on the end of the element in an inclined direction, and the reflected light is received as a monitoring light. In this system, the inclined end form of the element must be determined to an extent such that the inclined end does not affect the main output light from the element, and thus there is a problem in practical utilizability of this system.
Japanese Unexamined Patent Publication No. 5-53086 discloses an optical device in which a light-receiving element is directly arranged on an optical waveguide element so that a portion of the output signal light from the optical waveguide can be directly received and monitored by the light receiving element. In this device, a means for fixing the light-receiving element must be installed on the optical waveguide element and since the fixing means-installation work, a work for connecting the fixing means to the light-receiving element and an operation for adjusting the connected light-receiving element are effected after the optical waveguide element is fixed to a container case, the above-mentioned works and operation are very difficult and the possibility of damaging the optical waveguide element by the above-mentioned works and operation is high.